Transition In La0 Research Articles

Phase transitions in La1−x CaxMnO3−x/2 manganites

The crystal structure parameters and magnetic and electrical properties of La1−x CaxMnO3−x/2 reduced manganites with 0≤x≤0.5 are established. These investigations contribute to the understanding of magnetic interactions in manganites without Mn4+ ions. It is found that these manganites show a long-range antiferromagnetic order up to x=0.09 and transform into spin glasses at 0.09<x≤0.35. The compositions in the range 0.35<x≤0.5 show a strong increase in the spontaneous magnetization and critical point associated with the appearance of spontaneous magnetization and can therefore be viewed as inhomogenious ferromagnets. The magnetic and crystal structure peculiarities of La0.5Ca0.5MnO2.75 are established by the neutron diffraction method. The strongly reduced samples show a large magnetoresistance below the point where the spontaneous magnetization develops. The magnetic phase diagram of La1−x CaxMnO3−x/2 is established by magnetization measurements. The magnetic behavior is interpreted assuming that the Mn3+-O-Mn3+ magnetic interaction is anisotropic (positive-negative) in the orbitally ordered phase and isotropic (positive) in the orbitally disordered phase. Introduction of the oxygen vacancies changes the magnetic interaction sign from positive to negative, thereby leading to a spin glass state in strongly reduced compounds. The results obtained reveal unusual features of strongly reduced manganites such as a large ferromagnetic component, a high magnetic ordering temperature, and a large magnetoresistance despite the absence of Mn3+-Mn4+ pairs. In order to explain these results, the oxygen vacancies are supposed to be ordered.

Percolation nature of the metal-to-insulator transition in La0.8Ba0.2MnO3

For a La0.8Ba0.2MnO3 single crystal, the temperature dependences of its resistivity, magnetization, and differential magnetic susceptibility are studied in the vicinity of the metal-to-insulator transition and near the ferromagnetic-paramagnetic phase transition. The critical temperatures corresponding to these transitions are determined independently from the experiment. These temperatures are found to coincide within 2–3%. The results are discussed within the framework of percolation theory.

Paramagnetic behavior and correlation between high- and low-temperature structural and magnetic transitions inLa1−xSrxMnO3(x∼1/8)single-crystal perovskites

Paramagnetic behavior and correlation between high- and low-temperature structural and magnetic transitions in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mi>−</mml:mi><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Sr</mml:mi></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">MnO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mo>(</mml:mo><mml:mi>x</mml:mi><mml:mi>∼</mml:mi><mml:mn>1</mml:mn><mml:mo>/</mml:mo><mml:mn>8</mml:mn><mml:mo>)</mml:mo></mml:math>single-crystal perovskites

Spin-state transition in La1−xSmxCoO3 perovskites

Structural and magnetic properties of La1−xSmxCoO3 with x=0, 0.02, 0.04, 0.06, 0.08, 0.12, 0.16, and 0.2 have been systematically studied. LaCoO3 exhibits a rhombohedral structure of R3̄C symmetry. The lattice parameters decrease progressively with the incorporation of Sm, and a second orthorhombic phase of Pbnm symmetry appears for x⩾0.12. The volume fraction of the orthorhombic phase is a sensitive function of the Sm content in the compounds, and increases from 16% for x=0.12 to 75% for x=0.2. Accordingly, the low-spin-to-high-spin transition of the trivalent Co ions, characterized by a steep rise of the magnetization, shifts to higher temperatures. A molecular field analysis indicates an increased energy gap, from 317 K for x=0 to 500 K for x=0.2, between the low-spin and the high-spin state, implying a stabilization of the low-spin trivalent Co ions by the presence of Sm. The enhanced compressive chemical pressure due to the introduction of Sm is suggested to be responsible for the present observations.

Internal Chemical Pressure Effect and Magnetic Properties of La0.6(Sr0.4−xBax)MnO3

The effect of isovalent chemical substitution of Ba2+ into the Sr2+ sites on the structural and magnetic transition in La0.6(Sr0.4−xBax)MnO3 (x=0–0.4) has been investigated. The rhombohedral structure with space group R−3c is observed in this series of materials. Substitution of Sr and Ba in the La site changes the structural parameters such as the Mn–O bond lengths, thus allowing the study of the effect of the structure on magnetic properties. An increase in the Mn–O bond distance with increasing Ba content leads to a decrease in TC (the temperature of the paramagnetic to ferromagnetic transition).

Features of the rhombohedral-to-orthorhombic structural phase transition inLa1−xSrxMnO3

Features of the rhombohedral-to-orthorhombic structural phase transition in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mi>−</mml:mi><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Sr</mml:mi></mml:mrow><mml:mrow><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">MnO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Thermal-expansion and magnetostriction anomalies in phase transitions in La1−x SrxMnO3

A study is reported on phase transitions in the La1−x SrxMnO3 system, both spontaneous and induced by a pulsed magnetic field of up to 250 kOe, accompanied by anomalies in magnetoelastic properties. The temperatures of the polaron (charge) and magnetic ordering, as well as those of structural transitions, are observed to be in good agreement with the results obtained by other methods. Jumps in the field dependence of longitudinal and transverse magnetostriction associated with field-induced orbital ordering have been found. A strong temperature dependence of the corresponding threshold fields is observed.

Elastic-moduli and ultrasonic-attenuation anomalies near antiferromagnetic phase transitions in La1−xCaxMnO3

Both the longitudinal and transverse ultrasonic sound velocities and attenuations in the single-phase polycrystalline compound La1−xCaxMnO3 (x=0.63,0.83) have been measured by a conventional pulse-echo-overlap technique at a frequency of 10 MHz, between 50 and 300 K. The variations with temperature of the Young modulus E, shear modulus G, bulk modulus B, Poisson’s ratio σ, and attenuation α have been determined. Anomalies in E, G, B, σ, and α were observed near the temperature of the charge-ordering transition TCO and antiferromagnetic (AFM) transition TN. The results imply a very strong spin–phonon interaction caused by linear magnetostriction near the AFM transition. The detailed form of the anomalies is different for different types of phase transitions.

Magnetostriction study of structural and magnetic transitions in La1−xSrxMnO3 (0.1&amp;lt;x&amp;lt;0.2)

Magnetostriction measurements were performed for stoichiometric La1−xSrxMnO3 samples with x=0.11, 0.13, and 0.165 in magnetic fields to 12 T. By combining temperature and magnetic field dependent magnetostriction data with magnetization and resistivity results, the complex relationship between structural and physical properties was clarified. In the paramagnetic phase, large magnetic fields suppress a transition near x=0.13 from a structure with disordered Jahn–Teller (JT) orbital ordering (O*) to a structure with large coherent JT orbital ordering (O′), and near x=0.165 remove a transition from a structure with no coherent JT orbital ordering (R) to O*. The reentrant structural transition from O′ to O* appears between ferromagnetic and spin–canting transitions for x=0.11 and 0.13; temperatures of all transitions increase with magnetic field. The low temperature structural O* to O′ transition in the ferromagnetic phase for x=0.165 remains unchanged by application of an external magnetic field.

Epitaxial strain induced metal insulator transition in La0.9Sr0.1MnO3 and La0.88Sr0.1MnO3 thin films

We are reporting an unexpected metal insulator transition at the ferromagnetic phase-transition temperature for thin films of La0.9Sr0.1MnO3 (&amp;lt;50 nm), grown on a (100) face of SrTiO3 substrate. For the thicker films (&amp;gt;50 nm), similar to the single crystal, no such transition is observed below TC. Additionally, we observe the suppression of the features associated with charge or orbital ordering in intentionally La-deficient thin films of La0.88Sr0.1MnO3 (&amp;lt;75 nm). In thin films, transmission electron microscopy reveals a compressive strain due to the epitaxial growth, that is, lattice parameters adopt those of the cubic lattice of SrTiO3. As the film thickness increases, coherent microtwinning is observed in the films and the films relax to a orthorhombic structure.

Structural and magnetic phase transitions in La0.9Sr0.1MnO3

A study is made of phase transitions in doped La0.9Sr0.1MnO3 compounds using combined x-ray, electrical, and magnetic measurements. Structural phase transitions are observed accompanied by a change in the cell volume at temperatures of 100–110 K and 300–340 K. These structural changes are found to be related to different contributions of the rhombic Jahn-Teller Q 2 mode to the formation of the crystal lattice. The structural transition at 100–110 K is accompanied by distinctive magnetic and electrical properties. The data are analyzed in detail.

Modeling of a structural phase transition in La2−x SrxCuO4

The structural, vibrational, and elastic properties of La2CuO4 are calculated using a model for calculating the energy of the crystal based on interionic potentials with the multiparticle Jahn-Teller contribution included explicitly. The microscopic reasons for the structural instability of the La2CuO4 lattice relative to rotations of the oxygen octahedra are investigated. A structural phase transition from the orthorhombic phase (space group D2h18) into the tetragonal phase (space group D4h17) under hydrostatic compression of an La2−xSrxCuO4 crystal is modeled. The (P,x) structural phase diagram for La2−xSrxCuO4 is constructed.

Spatial cross-over of polarons across the CMR transition in La0.75Ca0.25MnO3 system.

Spatial cross-over of polarons across the CMR transition in La0.75Ca0.25MnO3 system.

Magnetic and structural transitions in La1−x Sr x MnO3: T-x phase diagram

The electrical conductivity, magnetic susceptibility, magnetization, and submillimeter (v=5∓20 cm−1) permittivity and dynamic conductivity of La1−x Sr x MnO3 (0≤x≤ 0.45) single crystals are investigated. The anomalies in the temperature dependences of these quantities are identified with diverse magnetic and structural phase transformations, including antiferromagnetic and ferromagnetic ordering, structural transitions between strongly distorted (Jahn-Teller) and weakly distorted (pseudocubic) orthorhombic phases, structural transitions to a rhombohedral phase and unusual transitions to a polaron-ordering state. As a result, the complete T-x phase diagram of the system La1−2x Sr x MnO3 is constructed in a wide interval of temperatures T=4.2∓1050 K and concentrations x=0−0.45.

Magnetic, dielectric, and transport properties of La1−xSrxMnO3 at submillimeter wavelengths

Antiferromagnetic resonance (AFMR), dynamic conductivity, and dielectric permittivity have been studied in the La1−xSrxMnO3 perovskites at a frequency range of ν=3–33 cm−1 by the use of a quasi-optical backward-wave-oscillator technique. Two AFMR modes have been observed in LaMnO3 and La0.95Sr0.05MnO3. Temperature dependencies of their resonance frequencies, linewidths, and mode contributions to a static magnetic permeability have been obtained and corresponding parameters of magnetic interactions have been extracted. A jump in the dielectric permittivity ε′(T) was found at T=100–120 K in La0.9Sr0.1MnO3, which indicates the existence of an additional phase transition (probably, a polaron ordering) besides the ferromagnetic transition at 170 K. A sharp increase of the dynamic conductivity σ′(ν,T) was observed near Curie temperature (285 K) at a metal–semiconductor transition in La0.825Sr0.175MnO3, which was slightly smaller than its static value.

Magnetostriction and field induced transitions in La1−xSrxMnO3 in pulsed magnetic fields

Magnetostriction λ∥(H) and field-induced phase transitions were studied in the single crystal of La1−xSrxMnO3 (x=0, 0.05, 0.1, and 0.175) in the pulsed magnetic fields up to 230 kOe. Anomalies in the λ∥(H) curves were revealed at 200 kOe in the LaMnO3, which were identified with the spin-reorientation AyFz→AzFy in the magnetic field H∥b axis. A complicated magnetostriction behavior was observed in La0.875Sr0.175MnO3, related to the field-induced transition between orthorhombic and rhombohedral crystal structures.

Simultaneous electronic and magnetic transitions in La1−xKxMnO3 thin films

We have fabricated perovskite La1−xKxMnO3 (x=0.14 and 0.2) thin films on SrTiO3 (100) by e-beam/thermal coevaporation. From the electrical and magnetic studies, we found a simultaneous occurrence of the paramagnetic to ferromagnetic state and insulating to metallic state, as predicted by the double exchange theory. The magnetoresistance is relatively small compared to that of the divalent cation doped La1−xAxMnO3 in fields up to 5 T.

New aspect of low-temperature phase transitions of La-cuprates

The change in the microstructure on aging at 12 K below a low-temperature structural phase transition in La1.5Sr0.1Nd0.4CuO4 has been examined by transmission electron microscopy. From a seriesof 100 forbidden dark field images obtained inin situ observation, spotty-shape regions with a Pccn/LTT tilt of the CuO6 octahedron were found to be nucleated and grow inside a LTO domain, accompanying the annihilation of some spotty-shape regions. In addition, there exists a tweed texture in fundamental dark field images, which is due to the macroscopic strain involved in the transition. Because a mapof the tweed texture is different from that of the spotty-shape region, the tilt of the octahedron is not always coupled to a proper spontaneous strain.

Optical spectroscopy of the charge-ordering transition in La1.67Sr0.33NiO4.

Optical spectra of a single crystal of ${\mathrm{La}}_{1.67}$${\mathrm{Sr}}_{0.33}$${\mathrm{NiO}}_{4}$, which undergoes a charge-ordering transition at ${\mathit{T}}_{\mathrm{CO}}$\ensuremath{\sim}240 K, have been investigated for wide ranges of photon energy (0.008--3 eV) and temperature (10--480 K). The opening of a charge gap as well as change of the gap magnitude (2\ensuremath{\Delta}) with temperature is clearly observed below ${\mathit{T}}_{\mathrm{CO}}$, with an anomalously large ratio of 2\ensuremath{\Delta}(T=0)/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathrm{CO}}$ (\ensuremath{\sim}13). Conspicuous spectral change with temperature is also observed above ${\mathit{T}}_{\mathrm{CO}}$ over the energy region up to 2 eV, which suggests persistent fluctuation of the charge ordering or formation of small polarons above ${\mathit{T}}_{\mathrm{CO}}$. \textcopyright{} 1996 The American Physical Society.

Some Factors Influencing Metal‐Insulator Transitions

AbstractThree problems are studied. (i) The optimal shape of the electronic densities of states of metal and gapped insulator related by a geometrical distortion, for a temperature induced transition between them. (ii) The dependence of valence band width on cation identity and the temperature of the metal Mott‐Hubbard insulator transition in the series RENiO3 (RE = Rare Earth). (iii) Electronic insights into the loss of a Jahn‐Teller distortion at Mn during the insulator‐metal transition in La1−xSrxMnO3 as x increases but not at Cu for La2−xSrxCuO4.